1. Field of the Invention
The present invention relates, in general, to a catalyst for the production of 1,1,1,2-tetrafluoroethane (CF.sub.3 CH.sub.2 F: hereinafter referred to as "HFC-134a") and, more particularly, to a catalyst for the partial fluorination of 1,1,1-trifluoro-2-chloroethane (CF.sub.3 CH.sub.2 Cl: hereinafter referred to as "HCFC-133a") to HFC-134a, significantly improved in durability, selectivity and activity. Also, the present invention relates to a method for the preparation of the catalyst.
2. Description of the Prior Art
Dichlorodifluoromethane (CCl.sub.2 F.sub.2 : hereinafter referred to as "CFC-12") has been one of the most important cooling agents utilized extensively in refrigerators, automobile cooling systems, and various other related industries because it is harmless to the human body and superior in thermodynamic physical properties. However, intensive research and observation has revealed that CFC-12 is a main substance that destroys the ozone layer in the stratosphere. According to the Montreal protocol internationally agreed in 1987, it is prescribed that CFC-12 should be prohibited from production and use starting from 1996.
HFC-134a, is another cooling agent that has similar physical properties to CFC-12. But, it does little damages to the ozone layer and has a much less influence to the earth's greenhouse effect. Accordingly, as the best substituent for CFC-12, it attracts attention with keen interest.
HFC-134a may be produced by reacting various C.sub.2 compounds with HF, for example, by reacting HCFC-133a with HF as shown in the following reaction formula: EQU CF.sub.3 CH.sub.2 Cl+HF.fwdarw.CF.sub.3 CH.sub.2 F+HCl
However, it is known that the substitution of fluorine for chlorine in HCFC-133a is a very difficult reaction. Thus, an effective catalyst is required to promote this reaction. As previously known, chromium oxide (III) (Cr.sub.2 O.sub.3) is an effective catalyst for the partial fluorination of HCFC-133a and superior to other catalysts. In many a patent, techniques for the preparation of the chromium oxide are disclosed.
Chromium oxide (III) catalysts developed thus far, however, include many disadvantages that need to be improved, such as catalyst life span and selectivity. For example, since the partial fluorination reaction proceeds at high temperatures, e.g. above 350.degree. C., an organic material, a reactant for the partial fluorination reaction, is discomposed to deposit carbons on the catalyst. As a result, the catalyst is deactivated at a rapid rate. In order to retard the deactivation rate of the catalyst, a process of providing oxygen together with the reactants at a constant ratio is disclosed in many patents, for example, European Patent No. 0 328 127 and German Patent No. 2932934. However, this process is not a basic solution for preventing the deactivation of the catalyst. In addition, the provided oxygen oxidizes the HCl resulting from the partial fluorination reaction to generate a chlorine gas which, in turn, reacts with the reactants to yield many by-products which make the separation and filtration of HFC-134a difficult. Of those by-products, particularly CF.sub.2 CHCl (hereinafter referred to as "HCFC-1122") has a boiling point almost identical to that of HFC-134a, thus it is extremely difficult to separate HFC-134a from the by-products.
Chromium oxide, used as a catalyst for the fluorination, may be produced from a variety of chromium compounds, and is used alone or in an active carbon-supported or alumina-supported form.
Thus far, many methods for preparing the chromium oxide catalyst have been developed and disclosed in many patents. For example, in Japanese Patent Laid-Open Publication No. 53-105404, U.S. Pat. No. 4,153,675 and Canadian Patent No. 1117145, a process comprising treating a chromium compound (III), such as chromium chloride and chromium nitrate, with ammonia to give chromium hydroxide and sintering it at 200.degree. to 500.degree. C. is disclosed. Another process comprising pyrolyzing ammonium dichromate at 500.degree. to 650.degree. C. is suggested in European Patent Laid-Open Publication No. 0313061.
For the chromium oxide catalyst to be active in the fluorination reaction, conventionally, it is completely dried at 400.degree. C. for 5 to 10 hours under nitrogen gas and then pre-treated with HF at 200.degree. to 400.degree. C. for 1 to 10 hours, prior to being used in the fluorination reaction. The pretreatment with HF is to change a part of the chromium oxide catalyst into a catalytic form of chrome oxyfluoride which is believed to be effective in the fluorination reaction. During this pretreatment, water is produced as a by-product as shown in the following reaction formula: EQU Cr.sub.2 O.sub.3 +a HF.fwdarw.CrO.sub.x .multidot.F.sub.y +b H.sub.2 O
The water by-produced gives rise to significantly increasing the ability of the coexisting HCl and HF in a reactor to corrode the equipment including the reactor, having a serious effect on the durability of the equipment. In addition, additional equipment is required to eliminate the water during the separation of HFC-134a, the final product, therefore, making the overall production procedure for HFC-134a complicated. Furthermore, when unreacted reactants are recovered to be reused, incomplete removal of water therefrom is the main factor that degrades the activity and durability of the catalyst.
U.S. Pat. No. 4,129,603 discloses that chromium hydroxide is treated in steam, changed into a catalytic form of chrome oxyfluoride, and then used in production of HFC-134a. However, the selectivity of this catalyst is merely in the range of 91 to 95%. In addition, since HCFC-1122, which is hard to separate from the product due to its similar boiling point, is by-produced in a large quantity, there is the disadvantage that an additional reactor is necessary to remove it.
Germany Patent No. 29 32 934 suggests the use of chromium fluoride or chrome oxyfluoride as a catalyst for the partial fluorination reaction. At a reaction temperature of 400.degree. C. and at a mole ratio of 7.7, early production yield of HFC-134a by this catalyst is only 26%. After reacting for 44 hours without supply of oxygen, the catalyst shows activity of just 22%. In addition, since oxygen need to be continuously supplied to reactants, the reaction is likely to produce by-products which makes the separation of HFC-134a more difficult.
European Patent No. 0328127 shows that a metal catalyst impregnated in alumina is effective for the production of HFC-134a. In this patent, the reported advantage of using this catalyst in the HFC-134a production process does not result in an increase of by-products compared to the case where chromium oxide catalyst is used, even during concomitant supply of reactants and oxygen. But, the selectivity of the catalyst for HFC-134a is at best 93.7% and at worst 55.2%.